Allogeneic bone marrow transplantation (BMT) currently constitutes the major therapeutic treatment option for a number of hematological malignancies, inherited disorders of hematopoiesis and certain in-born errors of metabolism. For leukemias and other neoplastic diseases, it is now appreciated that an important benefit of allogeneic BMT is a graft-vs-leukemia (GVL) effect. Unfortunately, the more widespread application of allogeneic BMT remains severely limited by the complications of graft-versus-host disease (GVHD), the associated morbid effects of long-term immunosuppression and problems of failure of engraftment. The experiments described in this proposal are aimed at overcoming these obstacles by the transplantation of purified hematopoietic stem cells (HSCs). The rationale for this approach is that purified HSCs lack the mature elements which allow the graft to mount a response against the host, thus engraftment of such cells will not cause GVHD, and in the absence of GVHD the need for long-term immunosuppression will be eliminated. An important adjunctive benefit derived from the successful transplantation of allogeneic HSCs, is that the resultant chimeric individuals are immunologically tolerant to subsequent donor-matched cell infusions. Thus, in the context of HSC engraftment it will be possible to infuse populations of donor derived cells that have been selected to confer GVL effects (but not GVHD) without the problems of host resistance to these tumor suppressing cells. In previous studies from our laboratory we isolated and characterized a purified hematopoietic stem cell population from mice. These cells are capable of self-renewal, give rise to all blood lineages, and are approximately 2000-fold enriched for their ability to radioprotect lethally irradiated mice. We have recently successfully achieved engraftment of these cells across major immune barriers and have begun to define some of the requirements to achieve durable engraftment. The major goals of the experiments outlined in this proposal are as follows: (1) To understand the cellular and molecular basis of resistance to allogeneic HSC and bone marrow transplants; (2) to develop rationale based therapies that allow stable allogeneic HSC engraftment with limited host morbidity; (3) to characterize and clone the cell populations, as well as the receptors from cells that confer GVL, so that it may become possible to produce "customized" transplants for neoplastic diseases in the future. Research in Project V is related to experiments performed in other subprojects.